1. Technical Field of the Invention
The present invention relates to a field-winding type of synchronous machine, and more particularly relates to an improvement of a structure for supplying electric power to a rotor winding (hereinafter also referred to as a field winding or a short-circuit winding) in a rotor-winding synchronous machine in which DC current is passed through a rotor.
2. Related Art
A rotary electric machine whose rotor rotates in synchronization with a rotating field that has been produced by an armature current, i.e., an AC current flowing through a stator winding, is called a synchronous machine. Known synchronous machines include those having a rotor structure of magnet type, field-winding type, reluctance type, and a type in which the foregoing types are mixed.
The field-winding type of synchronous machines exert excellent practical utility in speed-variable type rotary electric machines used, for example, for generating power for automotive performances, in that there is no need to place expensive permanent magnets in a rotor core, there is no need to consider resistance to centrifugal force of the magnets, and torque and generated (induced) voltage can be freely controlled by field flux control.
Japanese Unexamined Patent Application Publication No. 07-095790, for example, suggests a field-winding type of synchronous machine, which has a structure with no brush, and facilitates torque control.
However, the field-winding type of synchronous machine disclosed in the above publication is one in which a multiphase AC current which is in synchronization with a rotor position and is amplitude-modulated by modulated waveforms at bias frequency, is supplied to a stator winding. Thus, a current having large ripple flows through a rotor field winding, raising a problem of producing large torque ripple.